THE PHYLUM PORIFERA 177 



bath sponges (Fig. 9.5), whose skeletons are familiar objects, are found 

 in warm shallow waters with a rocky bottom, including the Mediter- 

 ranean Sea, the Gulf of Mexico and the Caribbean. They are hooked 

 from the ocean bottom by poles having a pronged fork at the end. A 

 short stay on shipboard is enough to kill them, after which they are 

 left lying in shallow water until the flesh is decayed. Then they are 

 beaten, washed and finally bleached in the sun. All that remains is the 

 spongin network, whose many tiny interstices permit it to soak up a 

 large amount of water. The sponge fishery is limited by the rate of 

 reproduction and growth of the sponges. Many of the grounds have 

 been overfished, and the fishermen are beginning to experiment with 

 the cultivation of sponges. Sponges are cut into many small pieces that 

 are fastened to cement blocks and set out in the sea. They take many 

 years to reach marketable size. 



Some of the Demospongia live only upon other organisms. The 

 boring sponges settle as larvae onto the shells of oysters or clams, into 

 which the young sponge bores by dissolving the shell. It does not harm 

 the host directly, but as the shell becomes honeycombed and weakened 

 it eventually falls apart, and the host is rapidly consumed by predators. 

 Another group, the hermit crab sponges, settle on snail shells inhabited 

 by hermit crabs. They grow to a considerable size, eventually completely 

 covering the shell. As time passes the shell dissolves, leaving a snail- 

 shaped cavity in the sponge, still occupied by the hermit crab. Because 

 it is carried around the sponge is never buried by silt (always a danger 



Figure 9.5. The common bath sponge. Only the spongin skeleton remains. (Courtesy 

 of the American Museum of Natural History.) 



